Dobereiner triads

Doberiener's Triads

 There are 114 elements that are known at present. Some elements have similar properties whereas some others have completely contrasting properties.

You must have observed that in a grocery store, things are kept in an orderly manner. For example, soaps are stacked at one place while biscuits are kept separately at another place. Scientists too tried to arrange elements based on their properties. However, as more and more elements were discovered, it became increasingly difficult to arrange these elements.

Hence, scientists began to look for some pattern in the properties of these elements. Let us study in this part how famous scientists such as Johann Wolfgang Dobereiner and John Newlands arranged the elements discovered at that time.

In 1817, Johann Wolfgang Dobereiner, a German chemist, classified elements into groups based on their properties. He kept all elements having similar properties in one group. Most of his groups had three elements each. Thus, he called these groups as triads. He was the first person to illustrate the relationship between the atomic masses of elements and their properties.

Atomic mass is the sum of the number of protons and neutrons in an element.

He also gave a law known as the Law of Triads. It states that when three elements in a triad are listed in the increasing order of their atomic masses, the atomic mass of the middle element will roughly be the average of the atomic masses of the other two elements. This is demonstrated in the following animation.

The above example illustrates Dobereiner’s Law of Triads.

Similarly, this law can also be proved for the triads of other elements as shown below:

Other metals		Non-metals
Element	Atomic mass	Element	Atomic mass
Calcium	40	Chlorine	35.5
Strontium	88	Bromine	80
Barium	137	Iodine	127

Average mass of calcium and barium =

= 88.5

Average mass of chlorine and iodine =

= 81.2

In both cases, the average atomic mass of the middle element is approximately equal to the average atomic mass of the other two elements.

Hence, Dobereiner was able to identify only three triads from the elements known at that time as shown in table.

Li	Ca	Cl
Na	Sr	Br
K	Ba	I

Dobereiner’s triads

Limitations of Dobereiner’s classification of elements:

• All known elements could not be classified into groups of triads on the basis of their properties.
• Not all groups obeyed the Law of Triads. For example, nitrogen family does not obey the Law of Triads.

Nitrogen family
Element	Atomic mass
Nitrogen	14
Phosphorus	31
Arsenic	74.9
Average mass of nitrogen and arsenic = = 44.45 In this case, the mass of phosphorus is not equal to the average mass of the other two elements i.e., nitrogen and arsenic. Hence, the elements of this group do not obey Dobereiner’s Law of Triads.

Although Dobereiner tried to classify elements into groups based on their properties, his classification was not very useful.

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